1. Field of the Invention
The invention relates to packaging of circuit board devices, and, in particular, to circuit board device magnetic and electric field shielding.
2. Discription of Related Art
Motherboard designers today are faced with tighter electromagnetic compatibility (xe2x80x9cEMCxe2x80x9d) regulations than previously. The Federal Communications Commission (xe2x80x9cFCCxe2x80x9d) recently imposed the so-called xe2x80x9copen boxxe2x80x9d electromagnetic interference (xe2x80x9cEMIxe2x80x9d) testing criteria. The open box criteria transfers responsibility to the manufacturer for the regulation of computer system (i.e., unintentional radiator""s) radiated energy produced by on-board components, such as application specific integrated circuits (xe2x80x9cASICsxe2x80x9d), from a shielded enclosure (e.g., a computer case or enclosure) housing the components to the internal components themselves, specifically a motherboard. Such emissions, which previously could be sufficiently attenuated by the shielded enclosure to satisfy the prior xe2x80x9cclosed boxxe2x80x9d regulations, must now be sufficiently attenuated without the shielded enclosure being completely closed. The exposed motherboard must thereby satisfy the open box regulations without relying on the shielded enclosure to provide sufficient attenuation. Otherwise, a given motherboard can only be marketed with a particular case if, when closed, the case reduces the emissions sufficiently.
With the open box criteria, the FCC changed the procedure for testing computer devices for radiated emissions. The FCC""s regulations were incorporated in amendments to xc2xa715.32(a)(1) of Title 47 in the Code of Federal Regulations (xe2x80x9cCFRxe2x80x9d). These amendments were adopted for CPU boards or motherboards and power supplies. Because of difficulties associated with determining the efficacy of shielding with computer cases, the FCC did not adopt rules that authorize these enclosures. To ensure that computer systems assembled from modular components comply with the technical standards, the FCC adopted a two-step test procedure for authorizing the motherboards. The motherboard must first be tested when installed in a typical enclosure but with the enclosure""s cover removed so that the internal circuitry is exposed at the top and on at least two sides of the enclosure. Other components, including a power supply, peripheral devices, and subassemblies are to be added, as needed, to complete the personal computer system. If the oscillator and the microprocessor circuits of the computer system are contained on separate circuit boards, both boards must be used in the test. Under this test procedure, radiated emissions from the system may be no more than 6 decibels (xe2x80x9cdBxe2x80x9d) above the limits specified in xc2xa715.109. These limits are shown in Table I below.
The testing is to be performed in accordance with the procedures specified in the measurement standards of xc2xa715.31. If the initial test shows that the open box computer system exceeds 6 dB above the limits shown in Table I, a further test is performed using the same configuration, but with the enclosure completely closed with all covers installed. Under these test conditions, the computer system under test shall not exceed the radiation limits specified in xc2xa715.109 of the rules. However, if the first test demonstrates that the computer system is in compliance with the radiation emission standards in xc2xa715.109, it is not required that the additional test be performed. The system must also be tested for compliance with the AC power line conducted limits as specified in xc2xa715.107, in accordance with the specified procedures in xc2xa715.31. If emissions greater than 6 dB above the limits can be identified and documented as originating from components other than the motherboard, then these emissions may be dismissed.
The test procedure of xc2xa715.32(a)(1) must be passed. Passing the first of the above tests, but failing the second, signifies a noncompliant product. If compliance cannot be demonstrated under the second test, then an alternative testing procedure is available in which the motherboard may be tested for compliance within the limits of xc2xa715.109 using a specified enclosure with the cover installed. Such testing must also be in accordance with the procedure specified in xc2xa715.31 and the motherboard that complies with the limits of xc2xa715.109 must be marketed together with the specific enclosure used for the test.
Reference is now made to FIG. 1 which shows a system 50 for encapsulating or covering a conventional device 12 (e.g., a chip) on a device board (e.g., a component board, a circuit board, a printed circuit board or PCB, a CPU board, a motherboard, and the like) known in the art. The encapsulation is integrated into the device packaging materials. One exemplary type of board that the device board could be is the motherboard of a computer system. In FIG. 1 the device board includes a surface 10 on which the device 12 is mounted as is well known in the art. The device 12 may be an integrated circuit component (or silicon die). The device 12 could be an ASIC, for example, a clock source, in addition to other types of devices. The device 12 may be coupled to conductive leads or components (not shown) by bond wires 14 through contacts 16 and 18 on or in the surface 10 and the device 12, respectively. The contacts 18 may be conductive contacts, ohmic contacts, Schottky barrier contacts, and the like, depending on the specific implementation of the system 50.
A xe2x80x9cprimaryxe2x80x9d layer 20 typically covers or encapsulates the device 12, the bond wires 14, and the contacts 16 and 18. However, due to other factors, for example, air bubbles or other imperfections, or by design, the primary layer 20 may only cover a portion of the device 12, the bond wires 14, and the contacts 16 and 18. The primary layer 20 is nonconductive and is formed from an industry standard encapsulant that is typically chemically resistant and thermally stable. The primary layer 20 may be, for example, an epoxy cresol novolac polymer (provided by Plaskon Singapore [a division of Amoco Chemical], Shinetsu, Nitto Denko, or others), or other polymer. Such nonconductive polymers are typically used in packaging material for integrated circuits, and they may be composite polymer matrix materials having various components. The primary layer 20 serves to protect the device 12 from possible oxidation, and to help maintain the structural integrity of the device 12, the bond wires 14, and the contacts 16 and 18, as is known in the art. However, the primary layer 20 offers no magnetic shielding or electric shielding for the device 12 of emitted (or received) radiated energy, and will not aid a given system in achieving compliance with the FCC open box regulations.
Thus, a need exists to shield device board components using device-appropriate shielding to facilitate compliance with the open box regulations instituted by the FCC. In the long term, this could simplify and reduce the cost of enclosing computer systems, perhaps allowing the use of plastic or other casing not constructed of conductive material.
In one aspect of the invention, a system 50 encapsulating a device on a device board having a ground is provided. The system includes a first polymer layer that covers a portion of the device and a conductive material that covers a portion of the first polymer layer and is coupled to the ground. The system also includes a second polymer layer that covers a portion of the first polymer layer and the conductive material.
In another aspect of the invention, a method of encapsulating a device on a device board having a ground is provided. The method includes covering a portion of the device with a first polymer layer and covering a portion of the first polymer layer with a conductive material coupled to the ground. The method also includes covering a portion of the conductive material and the first polymer layer with a second polymer layer.